Biodegradable polymers have been used to prepare biodegradable polymeric matrices that can be associated with, or formed into, implantable medical devices. For example, biodegradable polymers can be used to make a thin coating on a medical device's surface, generally designed to enhance the function of the device. Biodegradable polymers having thermoplastic properties can even be molded or formed into a shape to provide an implantable device having a structural property useful for treating a medical condition at the site of implantation. In theory, the polymeric matrix becomes totally degraded in the body. This can be advantageous for various medical applications, for example, such as to eliminate the requirement for explantation of the implanted article.
Implantable articles formed of or associated with biodegradable polymeric matrices can also be used to modulate the delivery of drugs to a patient at the site of implantation. Drug-releasing biodegradable matrices can be in the form of a coating on a device, or in the form of an implantable or injectable article that is formed primarily of the biodegradable polymer. Drug contained within the biodegradable matrix can be released or eluted from the matrix after the article has been introduced into the body.
Although there is a considerable amount of information regarding the use of biodegradable polymers for implantable medical devices, this field remains very technically challenging from a number of standpoints. For example, although biodegradable polymers should have properties suitable for the formation of a polymeric matrix in a desired form (such as a coating or a microparticle) it is often difficult to prepare such forms using conventional biodegradable polymers.
Also, the biodegradable polymeric matrix should be biocompatible, as well as the products that it degrades into. The polymeric matrix should not elicit a body response that adversely affects its intended function, such as a negative tissue response (e.g., a prolonged inflammatory response) at the site of implantation. Poly(lactide) and poly(glycolide) have considerable use as biodegradable polymers for implantable devices, but there are concerns regarding the amount of acidic degradation products generated upon their hydrolysis.
Another challenge relates to the actual biodegradability of the matrix that is implanted or injected in the body. Although many biodegradable polymers exhibit biodegradability in in vitro systems, they may not degrade in matrix form after the matrix has been introduced into the body. In other words, the chemical or physical property of the matrix may obstruct chemical or enzymatic activity required for degradation of the polymer (degradation that is otherwise seen when the polymer is free in solution and subjected to these degrading chemical or enzymatic activities). Accordingly, a lack of biodegradability of the polymeric matrix can diminish or negate the intended function of the device.
Also, some matrices degrade by bulk erosion. Bulk erosion may result in the loss of portions of the matrix after implantation and may cause an embolic event. In systems designed to release a drug, bulk erosion may result in loss of control over the drug release from the matrix.